The objective of the proposed work is to perform feasibility studies leading to the development of a novel, new detector for scintigraphy mammography capable of improving the diagnosis of breast cancer and lowering the number of false-positive diagnoses leading to unnecessary invasive biopsies, thereby reducing trauma to the patient while at the same time saving billions of dollars in the national medical bill that are now associated with these procedures. The concept is based upon a gamma- ray detector which combines a CsI(Tl) scintillator array coupled to a new Si-PIN diode photodetector array and indium-bump bonded to a large-scale low-noise integrated readout chip to form a high energy-resolution and high spatial-resolution imaging detector. The proposed development will lead (in Phase II) to a compact device which will include a high resolution collimator, CsI(Tl) scintillator and PIN photodiode array intimately coupled to the high-density low-noise electronics. The development will proceed through two distinct phases. In phase I we will design and fabricate a prototypical four-by-four (16-pixel) Si-PIN diode array and investigate the technological requirements necessary for fabricating and assembling the scintillator and for integration of the scintillator with the photodiode array. We will interface the array with an already existing 16-channel readout chip containing low-noise preamplifiers and amplifiers. We will evaluate the performance of the prototypical structure and compare it with the record results (7.5% FWHM energy resolution) obtained in preliminary studies on a 2x2x4mm CsI(Tl) scintillator coupled with a 2x2mm PIN photodiode. In phase II we will develop a scaled-up and optimized device with at least 625-pixels on a single ca. 5 cm by 5 cm chip and develop the technology for integrating all of the front-end components. We will also develop specific new readout electronics with 625 full channels of the optimized low-noise preamplifier, optimized amplifier, and logic circuits required for multiplexing the signals to the MCA on a single chip. We will also investigate packaging of the device and requirements for mosaicing the devices together to form larger focal plane arrays. PROPOSED COMMERCIAL APPLICATION: In the THIRD phase we will develop commercial detector systems based upon the Phase 1 and Phase 2 efforts. These new detectors could lead to a new generation of clinical imaging devices for improved mammography. Beyond the application in conjunction with mammography, the device under development is a precursor to much larger cameras with better spatial and spectral performance than any other camera in use today, and at lower cost. There is a very large market for this technology which is driven by the need for improved diagnostic tools for nuclear medicine.